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1 . Geologccal rec"Cs Inc Page 3 George's Service Feasibility Study Project No 425 2 April 28,2004 ' The shallowg roundwater would also cause a problem by mounding in and around the vapor extraction wells ' • The majority of the contamination is tied up between 7 and 28 feet bgs It would be very difficult to target this zone, even the upper most portion, as most of the year it is under the water table and the rest of the year it is within the capillary fringe zone ' 2.3 Monitored Natural Attenuation Monitored natural attenuation (MNA) consists of long term monitoring of groundwater conditions Attenuation of a groundwater plume can occur through physical processes such as advection and dispersion, or through biological processes In theory, biological organisms that can degrade hydrocarbons are ubiquitous in the subsurface These ' organisms metabolize the hydrocarbons in oxidation-reduction processes Under NINA, the laboratory analyses include chemical parameters that indicate whether ' oxidation-reduction reactions are taking place and natural biological mechanisms are decreasing the mass of residual contamination at a site The chemical parameters may include dissolved oxygen, sulfate, nitrate, oxidation-reduction potential, conductivity and pH A decrease in residual mass can be monitored by measuring concentrations of these parameters, along with a demonstrated decrease in contaminant concentrations in the sites' wells In essence, monitored natural attenuation involves monitoring the site over a period of several years to determine if the VOCs are degrading over time Intrinsic bioremediation is an option when the naturally occurring rate of contaminant biodegradation is faster than the rate of contaminant migration Unenhanced bioremediation is not a viable option for the site ' for several reasons • MtBE is a contaminant of concern • Sensitive receptors are less than 500 feet downgradient / ' • The dissolved hydrocarbon plume may still be migrating • Free product remains in the soil at the site ✓ ' • Low dissolved oxygen levels Most wells have dissolved oxygen levels less than I ppm, suggesting that anaerobic conditions may exist In intrinsic bioremediation the rate- controlling step is frequently the influx of oxygen When natural oxygen supplies ' become depleted, the microbes may not be able to act quickly enough to contain the contamination • Negative ORP Redox potential measurements are due to complex interactions between ' chemical species present in the groundwater and microbial byproducts This complexity makes ORP not the best field data for determining microbial activity However, negative ORP suggests anaerobic microbial processes will probably occur • The site has been under investigation for nearly 9 years and the groundwater concentrations remain relatively stable at the site ' 2.4 Enhanced Bioremediation Enhanced biodegradation can be achieved by the addition of microorganisms, oxygen, and nutrients to the subsurface environment to accelerate the natural biodegradation process